EXCEED THE SPACE PROVIDED. The long term goal of the proposed experiments is to understand how animals perceive odors. We plan to approach two aspects of this question. First, what is the animal's perception of the odorant and second, how does olfactory processing accomplish this perception. We propose to make activitymaps at three early stages in the olfactory processing pathway. First, maps of the input signals that are sent from the olfactory epithelium to the olfactory bulb. Second, maps of the input signals after they are sculpted by presynaptic inhibition acting on the olfactory receptor nerve terminals (processed-input). And third, maps of the mitral/tuftedcell output from the bulb. The first two kinds of maps will be made by imaging changes in calcium concentration in the olfactory receptor nerve terminals. We propose to use and/or develop four different kinds of activityindicators for measuring the mitral/tuftedcell output maps. The processed-input maps we have generated in the turtle and mouse show that individual odorants activate a large number of glomeruli and that different odorants activate overlapping, but distinct,glomerular populations, In the turtle the normalized processed-input maps are largely concentration invariant.In contrast, in the mouse the number of activated glomeruli increases dramaticallywith increasing odorant concentration. Unfortunately, the animals'perception of different concentrations of these odorants is unknown.We propose behavioral experiments to determine how rodents perceive different concentrations of an odorant. The experiments proposed here would further our basic understanding of olfactory processing. In the long term this understanding will provide opportunities for alleviatingfailures of processing of this important sensory input.